1. Field of the Invention
The present invention relates to a linear motor of a moving coil type provided with a stator having a field magnet as well as a movable piece which has an armature coil opposed to the field magnet and is movable along the stator. Also, the present invention relates to an image reader using a linear motor of a moving coil type for optically scanning and reading an original image and, more specifically, for linearly driving a slider which carries optical parts including an illumination lamp.
2. Description of the Background Art
Linear motors have been utilized for linearly moving objects in a wide field including office automation equipments such as copying machines, printers and image scanners, factory automation equipments such as X-Y tables and object transporting devices, and optical equipments such as cameras.
In one of known types of linear motors, a stator is provided with a field magnet having N-type magnetic poles and S-type magnetic poles, which are arranged alternately and linearly in a predetermined direction, and a movable piece, which is movable along the stator, is provided with an armature coil opposed to the field magnet. This type of linear motor is called a moving coil type.
In the linear motor of the moving coil type, the armature coil of the movable piece is energized under control to move the movable piece along the stator. This energizing of (i.e., power supply to) the armature coil of the movable piece as well as the control of energizing are performed based on information sent from various sensors which are arranged on the movable piece as will be described later. For these energizing and control, a harness (i.e., a bundle of electric cables) is extended from the movable piece to connect a power supply circuit for the armature coil with the armature coil and the various sensors on the movable piece.
The sensor for controlling the power supply to the armature coil is provided, e.g., for the field magnet, and more specifically is provided, e.g., for detecting a polarity of magnetic pole of the field magnet to which the moving armature coil opposes and/or for detecting an intensity of the magnetic field formed by the field magnet. As this sensor for the field magnet, a magnetoelectric conversion element such as a Hall element or a magnetic resistance element (MR element) is usually employed. The magnetoelectric conversion element can issue an electric signal corresponding to the polarity of the magnetic pole and the intensity of the magnetic field.
In addition to the above, a sensor for an encoder is employed as the sensor which is arranged on the movable piece for controlling the power supply to the armature coil. The encoder is formed of the encoder sensor (i.e., sensor for the encoder) arranged on the movable piece and an encoder scale which is arranged at a stationary position and extends in the lengthwise direction of the stator. The encoder may be of either an optical type or a magnetic type as is well known.
The encoder scale for the optical encoder is provided with two kinds of surfaces which have optically different properties and are arranged alternately in the lengthwise direction of the stator. For example, an encoder of a so-called reflection type is completed by employing two kinds of (e.g., white and black) surfaces which have different reflectances to each other and are arranged alternately in the lengthwise direction of the stator. An encoder of a so-called transparent type is completed by employing two kinds of surfaces which have different light transmittances to each other and are arranged alternately in the lengthwise direction of the stator. In either case, the sensor for the optical encoder includes a photoelectric conversion element such as a photodiode or a phototransistor, which can issue electric signal corresponding to quantity of light. In some cases, the sensor for the optical encoder may be a photosensor (an optical sensor) which is one packaged combination of a light emitting element such as a light emitting diode (LED), which emits light toward the encoder scale, and a photoelectric conversion element.
In the magnetic encoder, the encoder scale is provided with N- and S-type magnetic poles arranged alternately in the lengthwise direction of the stator. The sensor for the magnetic encoder usually may be a magnetoelectric conversion element such as a magnetic resistance element (MR element) or a Hall element which issues an electric signal corresponding to the polarity of the magnetic pole of the magnetic encoder scale and/or the intensity of the magnetic field.
A power supply and drive circuit for supplying a current to the armature coil and thereby driving the movable piece along the stator usually operates to supply the current to the armature coil based on a field magnet signal sent from the foregoing sensor for the field magnet and/or an encoder signal sent from the encoder sensor. For achieving a compact structure of the circuit, the power supply and drive circuit sometimes employs a motor drive IC. The power supply and drive circuit is arranged at a fixed position outside the movable piece.
The linear motor of such a moving coil type may be used in the image reader such as an image scanner for optically scanning and reading an original image. In the image reader, a slider carrying optical parts such as an illumination lamp is linearly moved for optically scanning the original image, and the linear motor is used for driving the slider. A fluorescent lamp is used as the illumination lamp in many cases, and a turn-on circuit for the illumination lamp is arranged at a fixed position outside the movable piece.
However, when the harness extended from the movable piece has many cables for energizing the armature coil on the movable piece, for transmitting the output signals from the sensors for the field magnet, for transmitting the output signals from the sensors for the encoder and others, it is difficult to handle and rout the harness having such many cables. The armature coil is usually formed of two or more coils. For example, when three-phase energizing is performed, the armature coil is formed of one or more coil group(s) each including three coils. In any of the structures, as the coils forming the armature coil increases in number, the cables extended from the movable piece increase in number so that smooth movement of the movable piece may be impeded, and/or routing of the harness may be difficult. When the harness is bent excessively, and thereby the harness is broken, the movable piece cannot be driven.
As described above, the magnetoelectric conversion element for the field element, photoelectric conversion element for the encoder and/or the magnetoelectric conversion element for the encoder may be arranged on the movable piece for controlling the power supply to the armature coil. In this case, each of electric signals issued from these elements are usually extremely weak analog signal so that the signal is liable to be affected by noises during transmission through the harness extended from the movable piece. If the signals issued from these elements are affected by noises, the control of energizing the armature coil cannot be performed precisely, and therefore the movable piece cannot be driven precisely.
Noise sources of such noises are, for example, as follows. The harness extended from the movable piece includes a cable for energizing the armature coil. This cable for energizing the armature coil transmits a current larger than signals or the like issued from the foregoing sensing elements, and is routed in parallel with the cables for transmitting signals issued from the foregoing elements in many cases. Therefore, the cable for energizing the armature coil forms the noise source.
In the image reader for optically scanning and reading an original image, the linear motor can be used for linearly moving the slider carrying optical parts, as described above. If the image reader is provided with a liquid crystal display for displaying various information, a display circuit of the liquid crystal display may form the noise source.
In the case where the linear motor is used, in the image reader, for driving the slider carrying optical parts including the illumination lamp such as a fluorescent lamp as described above, a cable for an illumination lamp turn-on circuit arranged at a fixed position outside the movable piece is likewise extended from the movable piece or a slider portion near the same. This cable further increases the number of cables of the harnesses extended from the movable piece or a portion near the same, which further makes the routing of the harness difficult. In the above image reader, if the movable piece is not driven precisely due to an influence by noises as described above, good image reading cannot be performed.